I'd say go with magnets on the bottoms of the seeker feet, they'd stay hidden almost all the time and would provide great strength for holding the leg on. Also, can you rotate the tailfins to overlap the SSS leg without forcing it too much? That could be another way to help hold the modded seeker on.

Using the tailfins to help sounds like a good idea. If you want the fins like they are then maybe you could scratch build a piece to use the small gap between the knee pads and the screw holes located there. Like a "T" shaped piece where the top of the T fits into the screw holes and the bottom of the T has a clip to attach to the shin piece.